Thin film deposition for semiconductor device fabrication is generally accomplished through a gas phase process, such as chemical vapor deposition (CVD), plasma enhanced deposition (PECVD) or atomic layer deposition (ALD). The process is typically carried out in a deposition chamber under vacuum conditions with the substrate being exposed to the precursor vapor at an elevated temperature to form thin film. If the precursor chemical is a liquid at room temperature the precursor liquid must first be vaporized to generate vapor to form the desired thin film.
Modern semiconductor device fabrication typically uses a liquid-source vaporization apparatus to generate vapor for thin film deposition. The most widely used vaporizer is the Direct Liquid Injection Vaporizer, which is also referred to as a DLI vaporizer because the liquid is directly injected into the heated vaporizer to generate vapor on demand, i.e. on an as needed basis.
The rate of gas and liquid flow into a DLI vaporizer must be accurately measured and précised controlled. Accurate liquid flow control is usually more difficult than the accurate control of gas flow due to the relatively low rate of liquid flow needed and some liquid properties, such as viscosity and surface tension that can affect the actual rate of liquid delivery into the vaporizer.
The rate of fluid flow can vary greatly from one application to another. In large industrial installations, the liquid flow rate can be in the kilogram per min or higher range. In semiconductor and laboratory research applications, the required liquid flow rate is generally much lower. The flow rate is typically less than 50 g/min and flow rate as low as 0.1 mg/min, i.e. 0.0001 g/min, or even lower may also be encountered. The method and apparatus of the present disclosure are particularly suitable for the control of such low liquid flow rates.